In electron beam drilling a material, called a backer, is required at the surface of the workpiece facing away from the beam source. In the absence of a backer a beam penetrating the exit surface may hit another surface of the workpiece, and more importantly, a hole drilled through the workpiece will be poorly shaped. The backer has the dual function of absorbing or dissipating excess electron beam energy and generating gaseous pressure when hit by the beam. The pressure must be sufficient to expel molten workpiece material from the hole being drilled in a direction opposite to that from which the beam comes; this action will produce a well shaped symmetrical hole.
For high performance applications it is not enough that the hole be well-formed; it is also necessary that there be a minimum thickness recast, or melted and resolidified, layer of metal which is typically found along the bore of the hole. It is also desired that there be a minimum of backer residue in the hole, since this may be hard to remove and undesired in actual use because of its properties.
The dual requirements of durability under beam action (to protect other portions of the workpiece or fixture) and controlled degeneration (to provide the necessary gaseous force) require that backer compositions be carefully chosen. Koenig, U.S. Pat. No. 3,649,806 describes a preformed metal and ceramic cellular structure backer containing volatile materials. U.S. Pat. No. 4,239,954 of Howard, Jordan and Yaworsky, (of common assignee herewith) entitled "Backer for Electron Beam Hole Drilling" discloses backers generally having ceramic particulates contained in a liquifiable binder; the disclosure thereof is hereby incorporated by reference. The particulates which are mentioned as useful include metal alloys, fine glass bead, glass frit, fused silica and alumina, calcia, magnesia, silica and zirconia powders. Soda lime glass is exemplified as being particularly useful. Binders which are disclosed include sodium silicate, silicone rubber, epoxy resins, waxes and polyvinyl alcohol.
The present invention is concerned with novel particulates which are includable in the backer. Previously, it was recognized that the backer had to have certain characteristics, namely the volatility and durability mentioned above, but it was not apparent how these characteristics were most desirably produced by the separate contributions of the particulate and binder. As the above mentioned U.S. Pat. No. 4,239,954 indicates, the properties of the binder are often times largely determined by the ease with which the backer may be applied and removed from the workpiece. A number of particulate materials have been stated as being useful in the prior art but not much distinction was made between there different particulates. Metal particulates are noted as being usable in the prior art, but generally the ceramic materials are preferred because of lower cost, their relative inertness, and their refractory properties insofar as stopping the beam. But, there is no technical teaching in the prior art to enable a choice among the ceramics. Glass of the soda lime type is preferred mainly because of its common availability and low cost. The characteristics of particulates have now been investigated more fully and have resulted in the discovery of very useful glass particulates which are claimed herein. The invention herein has relation to J. Brennan's application Ser. No. 239,275, "Alkali Metal Oxide Free Backers for Energy Beam Drilling" filed on even date hereof.